Synergists: Clavicular (Upper) Pectoralis Major, Anterior Deltoid, Biceps Brachii (short head only), Triceps Brachii (only with elbow extension; see comments)
Mechanics: Isolation or compound. See comments
Place two short barbells or EZ-curl bars close to, and parallel to, each other. Ensure that the barbells can roll.
Assume a push-up position over the barbells, holding each one in the center or a little above the center.
Keeping your body straight and rigid, inhale as you slowly roll the barbells away from each other and lower your body until you feel a mild stretch in your chest.
Exhale as you pull the barbells back together and return to the starting position.
Comments and tips
The bodyweight fly is an advanced exercise. Take it slowly and carefully. Begin by rolling out just a little. As you become stronger and more confident, increase the extent to which you roll out.
Fly exercises, such as the dumbbell fly, are isolation exercises (only a single joint is involved). The elbows are kept stiff, and all of the movement occurs in the shoulder joints. However, keeping your elbows stiff with the bodyweight fly can be difficult, especially when you first start to use the exercise. More than likely, you will have to flex and extend your elbows, at least a little, as you would with a push-up. If you do flex and extend your elbows, the exercise will be a compound movement, and your triceps brachii will act as synergists. On the other hand, if you do not flex and extend your elbows, the exercise will be an isolation movement, and your triceps brachii will only act as stabilizers. As you become more advanced at the bodyweight fly, try to eliminate elbow flexion and extension, thus taking the triceps brachii out of the picture and turning the exercise into a proper fly.